Question 1

In mass spectrometry, what is the sequence of events for a molecule from its introduction to its detection?

Accepted Answer

The molecule is ionized, fragmented, accelerated, separated by mass, and detected.

Question 2

2-Methylhexane shows an intense peak in the mass spectrum at $$m/z = 43. $$Which fragment is most likely responsible for this peak?

Accepted Answer

The butyl cation $$\mathrm{C_4H_7^+}$$

Question 3

An unknown compound has a molecular ion peak with a relative abundance of 43.27% and an $$M+1$$ peak with a relative abundance of 3.81%. How many carbon atoms are present in the compound?

Accepted Answer

Approximately 6 carbon atoms

Question 4

Which $$m/z$$ value is the base peak for benzyl alcohol, and why?

Accepted Answer

$$m/z = 91$$, due to the tropylium ion formation

Question 5

Which fragment is formed at $$m/z = 102$$ from the molecular ion peak of 2-hexanol?

Accepted Answer

Loss of water ($$\mathrm{H_2O}) $$from the molecular ion

Question 6

Which compound would be expected to show intense IR absorption at $$1746\ \mathrm{cm}^{-1}$$?

Accepted Answer

$$\mathrm{CH_3CO_2CH_3} (an $$ester)

Question 7

What is the expected IR absorbance for phenylacetylaldehyde?

Accepted Answer

A strong absorption near $$1700\ \mathrm{cm}^{-1}$$ due to the aldehyde $$\mathrm{C=O}$$ stretch

Question 8

Which fragment is formed at $$m/z = 45$$ from the molecular ion peak of 2-hexanol?

Accepted Answer

Loss of butyl group ($$\mathrm{C_4H_9})$$

Question 9

Which fragment is formed at $$m/z = 87$$ from the molecular ion peak of 2-hexanol?

Accepted Answer

Loss of methyl group ($$\mathrm{CH_3$})$$

Question 10

Which fragment is formed at $$m/z = 84$$ from the molecular ion peak of 2-hexanol?

Accepted Answer

Loss of ethyl group ($$\mathrm{C_2H_5})$$

Question 11

A mass spectrometer detects a base peak at $$m/z = 91$$ for a compound with the structure shown in the image. What is the most likely identity of this fragment?

Accepted Answer

Tropylium ion

Question 12

Which of the following best explains why esters show strong IR absorption near $$1746\ \mathrm{cm}^{-1}$$?

Accepted Answer

The $$\mathrm{C=O}$$ bond in esters is highly polar and absorbs strongly in this region.

Question 13

If a compound has a molecular ion peak at $$m/z = 108$$ and a base peak at $$m/z = 91$$, what does this suggest about its structure?

Accepted Answer

It contains a benzyl group that fragments to form a tropylium ion.

Question 14

A compound with the formula $$\mathrm{C_6H_5CH_2OH} is $$analyzed by mass spectrometry. Which fragment is most likely to be observed at $$m/z = 91$$?

Accepted Answer

Tropylium ion $$\mathrm{C_7H_7^+}$$

Question 15

Which of the following is a common misconception about the $$M+1$$ peak in mass spectrometry?

Accepted Answer

It is caused by the presence of $$^{13}\mathrm{C}$$ isotopes, and its intensity can be used to estimate the number of carbon atoms.

Question 16

Which of the following compounds would NOT show a strong IR absorption near $$1746\ \mathrm{cm}^{-1}$$?

Accepted Answer

$$\mathrm{CH_3CH_2OCH_2CH_3} (an $$ether)

Question 17

A compound is found to have a strong IR absorption at $$1700\ \mathrm{cm}^{-1}$$ and a mass spectrum base peak at $$m/z = 91. $$Which functional groups are likely present?

Accepted Answer

A benzyl group and an aldehyde group

Question 18

Which of the following best describes the fate of a molecule in a mass spectrometer?

Accepted Answer

The molecule is ionized, fragmented, accelerated, separated by mass, and detected.

Organic Chemistry Mass Spectrometry & IR Spectroscopy Study Guide

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